The present invention relates to a device for measuring optical characteristics of the atmosphere, and more particularly to a device which is especially intended for measuring optical charcteristics of the atmosphere of an airfield.
Devices for measuring optical characteristics of the atmosphere, for example fog, smog for the like, are already known. They are usually constructed as transmissometers or optical back scatter measuring devices. For further details reference may be had to U.S. Pat. Nos. 3,672,775; 3,808,430 and 3,323,409, in which devices of this general type are disclosed in detail.
Such devices are used, inter alia, to measure the visibility in the atmosphere over airfields. They emit and/or receive light pulses and operate on the principle of comparing the intensity of the received light pulses with the intensity of the emitted light pulses, the detected variation then being a measure of the visibility in that atmospheric region through which the light pulses have travelled. Current international air safety regulations require that the measurements must be taken (i.e. that the light beams must travel) at a certain height above the surface of an airfield, which height is currently 2.80 meters above the surface, because the cockpits of the presently most frequently flown aircraft are located at this height above the ground when the aircraft touches ground.
The devices of the prior art employ upright vertical tubes at or near the top of which the electrical and optical components are mounted which are required for emitting and/or receiving the light pulses. Thus, the major portion of the mass of this prior-art device is located at or near the current measuring level of 2.80 meters, which brings with it the disadvantage that in the event of the collision of an aircraft with one of these devices significant damage to the aircraft can be expected to occur since the aircraft will collide with that part of the device at which the largest portion of the mass of the device is located. This danger exists particularly in the case of smaller aircraft, such as recreational aircraft, small business jets and the like, which because of their relatively low weight can be more readily deflected off the runway by sudden lateral winds than the larger commercial craft.
A proposal has been made in the prior art to so mount these devices that if a colliding aircraft engages a feeler of the device, the device will flip away for which purpose it is hingedly mounted. However, given the landing speeds of the modern aircraft it will be understood that the reaction time of a device constructed in this manner is too long to avoid a collision and the resulting danger.
Moreover, th prior-art devices of the type in question have the further disadvantage that the protective cover, through which the outgoing or incoming light pulses travel, tend to become obscured because they are not protected against such a possibility. Once so obscured, the intensity of the outgoing and/or incoming light beams will be falsified and this will influence the measured results. The problem exists not only in the event of rain or snow, but also -- and particularly -- as the result of deposition of pollutants -- e.g. combustion products -- resulting from the engines of the aircraft themselves. The combustion gases emitted by the engines of modern aircraft during starting and landing carry with them extremely small aerosols which travel at high diffusion speed and tend to form a fine milky coating on optical surfaces; all proposals made heretofore in the prior art for preventing the formation of such coatings have failed.